I have two layers with different objects in an animation how do I render these two layers together?
To activate more than one layer, you need to Shift+LMB the layer buttons in the 3D-View Header or the Render Layers, or you can press the appropriate number on the top row of your keyboard (Shift+1, Shift+2,... for the top row, Shift+Alt+1, Alt+2... for the bottom) To select all press `.

The darker buttons are visible layers, and the lighter ones invisible.

The dots in the layer buttons indicate that they contain at least one object. The layer that contains the active object will have an orange dot instead of a grey one.



Documentation of Layer Settings